Polymers having alkenyl functional groups at their molecular ends are known to be crosslinked in combination with proper curing agents to produce cured compositions having high heat resistance, weathering resistance, etc. For example, they are crosslinked or cured with a compound having a hydrogen-silicon bond as a curing agent, or by using photoreaction.
Examples of known main chains for polymers having alkenyl end groups are polyethers, such as poly(ethylene oxide) and poly(propylene oxide); hydrocarbon polymers, such as polyisobutylene, polybutadiene, polyisoprene, polychloroprene, hydrogenated polybutadiene, and hydrogenated polyisoprene; and polyesters, such as poly(ethylene terephthalate), poly(butylene terephthalate), and polycaprolactone. These polymers are used in various applications depending on the main chain structures and the manner for crosslinking.
A vinyl polymer has a main chain including a carbon-carbon bond only and exhibits high weathering resistance. If an alkenyl group is incorporated into ends of a vinyl polymer chain, a curable composition having superior physical properties can be prepared compared to a polymer having alkenyl groups in the side chain. Although production methods therefor have been studied by many researchers, industrial production methods which are simple and easy have not yet been found.
Japanese Unexamined Patent Application Publication No. 5-255415 discloses a method for synthesizing a vinyl polymer having alkenyl groups at both ends using a disulfide having alkenyl groups as a chain transfer agent. Japanese Unexamined Patent Application Publication No. 5-262808 discloses a synthetic method in which a vinyl polymer having hydroxyl groups at both ends is produced with a disulfide having hydroxyl groups, and a vinyl polymer having alkenyl groups at both ends is produced by the reaction of the hydroxyl groups. However, in these methods, it is difficult to introduce alkenyl groups into both ends reliably, and it is not possible to prepare curable compositions having satisfactory characteristics. In order to introduce alkenyl groups into both ends reliably, a chain transfer agent must be used in large quantity, resulting in problems in the production process and cost.
In order to overcome the problems described above, methods are disclosed, such as a method in which alkenyl groups are incorporated into ends of a polymer using an organometallic compound (Japanese Unexamined Patent Application Publication No. 9-272714), and a method in which atom transfer radical polymerization is performed using an organohalide or the like as an initiator and a transition metal complex as a catalyst, and alkenyl groups are introduced by the terminal halogen atom (Japanese Unexamined Patent Application Publication No. 2000-128924). However, in these methods, since a large amount of metal compound is used, the resultant polymer may be colored, or the production process is complicated because of additional purification steps. In some cases, because the metal complexes inhibit hydrosilylation, a large amount of hydrosilylation catalyst is required.
On the other hand, reversible addition-fragmentation chain transfer polymerization (RAFT polymerization) is an excellent technique for producing a vinyl copolymer because the molecular weight and the molecular-weight distribution can be controlled and the technique can be applied to a wide variety of monomers and polymerization methods. The details of the technique including its reaction mechanism are described in PCT Publication Nos. WO98/01478, WO99/05099, and WO99/31144, and Macromolecules 1998, 31, 5559–5562. However, these documents neither disclose a method for introducing alkenyl groups into ends nor describe a curable composition. The present invention relates to a method for introducing alkenyl groups into ends by a RAFT polymerization technique, and a curable composition containing a vinyl polymer, as an essential component, having alkenyl groups produced by the RAFT polymerization technique.